Senile dementia is divided broadly into the Alzheimer type dementia and the cerebrovascular dementia, and about 80% of the patients of senile dementia can be classified into these categories. As the population rapidly ages, the number of the patients of senile dementia demonstrates an upward trend in these days. In Japan, it is speculated that about 7% of the people 65 years old or over show the symptoms of dementia, and hence, it is an urgent need to develop an excellent therapeutic agent for dementia. The Alzheimer type dementia is accompanied by senile plaque and neurofibrillary tangle, and it is pathologically characterized by encephalatrophy caused by significant neuronal death. In familial Alzheimer's disease, several gene mutations have been identified, whereby a leading hypothesis for neuronal pathogenetic mechanism thereof has been speculated, but the most of cases are sporadic, and hence, it may be said that Alzheimer's disease is still a disease of unknown cause. Accordingly, at the present, there is no radical therapeutic method for inhibiting neurodegeneration. The Alzheimer type dementia shows as core symptoms cognition dysfunctions such as disorders of memory, faculty of orientation, attention, etc., and it is also accompanied by peripheral symptoms such as psychotic manifestations or abnormal behavior problems (e.g., depression, aggressive attack, delusion, etc.). In the symptomatic treatment of these symptoms, only an acetylcholine esterase inhibitor has been clinically used, and it has been reported that acetylcholine esterase inhibitors are also effective to not only core symptoms but also peripheral symptoms. In the treatment with acetylcholine esterase inhibitors, neurotransmitter acetylcholine is supplemented by inhibiting acetylcholine-degrading enzyme, while acetylcholine neuronal cells, which are closely-linked with cognitive function, are especially disturbed in Alzheimer's disease and neurotransmitter acetylcholine is reduced.
On the other hand, the cerebrovascular dementia is a disease which develops owing to cerebrovascular disorders, and at the moment, there is no cure for core symptoms thereof. However, recently, the clinical trial of acetylcholine esterase inhibitors has been done, and it has become apparent that these medicaments are also effective to cerebrovascular dementia. Accordingly, there is a possibility that a therapeutic agent having a similar therapeutic mechanism to the Alzheimer's disease such as acetylcholine esterase inhibitors may be effective even to cerebrovascular dementia (e.g., Rinsho-Seishinigaku (i.e., Clinical Psychiatry), 31 (10): 1189-1193 (2002)).
On the other hand, there has not been known any therapeutic agent which shows no acetylcholine esterase inhibitory activity but is effective to senile dementia such as the Alzheimer type dementia and the cerebrovascular dementia. Moreover, JP Patent No. 2800953 discloses imide derivatives showing an excellent antipsychotic activity and anxiety reducing activity, but it has never indicated whether or not those derivatives show effects on senile dementia.